Multi-signal transmit array with low intermodulation

ABSTRACT

A transmitter is provided for simultaneously transmitting a plurality of signals in a plurality of directive beams to corresponding destination stations, each destination station located in a separate fan within a service area. The transmitter includes a plurality of beamformers, each beamformer receiving one of the signals to be transmitted to an associated fan, each of the beamformers having a plurality of outputs for each different signal to be transmitted. A plurality of Butler matrices each receive one of the plurality of outputs from the plurality of beamformers for each different signal to be transmitted, each Butler matrix having a plurality of outputs in phased relationship to one another, wherein each of the signals to be transmitted is simultaneously provided across the outputs of each Butler matrix in a phased relationship. An antenna is provided with an aperture within which a two-dimensional array of antenna elements are disposed, wherein equal fractions of adjacent antenna elements are connected to the outputs of each Butler matrix, and wherein each of the plurality of signals are simultaneously transmitted by the entire two-dimensional array of antenna elements. Each of the plurality of beamformers receives steering control signals for steering the direction of each beam within its respective fan.

This is a division of Application No. 09/055,490, filed Apr. 6, 1998 nowU.S. Pat. No. 6,377,588.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed toward an active phased arraytransmitter for transmitting multiple signals and, more particularly,toward an active phased array transmitter for simultaneous transmissionof multiple signals with a minimum of intermodulation distortion betweensignals.

BACKGROUND OF THE INVENTION

Transmitters having active phased array antennas, i.e., phased arraytransmitters, are commonly used in the transmission of signals. Activephased array transmitters can be configured to transmit signals inhighly directive beams by using multiple antenna elements each connectedto an elementary transmitter power amplifier. The signals are fedthrough each power amplifier and are relatively phased to achieveconstructive addition of radiation in the desired direction. When only asingle signal is to be transmitted in a single beam, class-C singlefrequency power amplifiers may be efficiently utilized. The transmittedspectrum can then be determined from the phase modulationcharacteristics of the information modulated on the signal.

Phased array transmitters may also be configured to transmit multiplesignals in multiple directed beams. In this case, combinations of themultiple signals to be transmitted are formed by a beamforming networkusing a set of beamforming coefficients. The signal combinations are fedto elementary transmit power amplifiers, with each power amplifierassociated with a respective antenna element or subgroup of antennaelements. In this case, the power amplifiers must be linear, multiplesignal power amplifiers, also known as Multiple Carrier Power Amplifiersor MCPA's. However, a problem with this prior art configuration is thatimperfect MCPA linearity causes nonlinear distortion or intermodulationbetween the signals generating unwanted signals. These unwanted signalsmay lie outside the allocated frequency band, potentially interferingwith other services. Even when each signal alone, and thus their linearcombinations formed by a beamformer, are band limited, the non-linearlyamplified combinations will exhibit out-of-band spectral components dueto the non-linear distortion.

Improvements to such prior art active phased array transmitters fortransmitting multiple signals in multiple beams while reducingintermodulation and improving efficiency are described in U.S. Pat. Nos.5,548,813; 5,555,257; 5,568,088; 5,574,967; 5,594,941; 5,619,210;5,619,503; and 5,638,024, all of which are herein incorporated byreference. The afore-listed patents are principally directed towardimproving the efficiency of use of a phased array antenna and/orimproving the intermodulation and efficiency performance of MCPA's usedin phased array antennas. While the afore-listed patents are notprincipally directed toward reducing intermodulation radiation inunauthorized frequency bands, they may provide guidance.

While a single-signal phased array for transmitting a signal in a highlydirective beam may of course be provided in multiple copies in order totransmit multiple signals, the total aperture area of the plurality ofsingle-signal antenna arrays is inefficiently used since only a fractionof the antenna elements are used to radiate each signal. For instance,if two single-signal phased array antennas were used, each would useone-half of the total antenna elements; if four single-signal phasedarray antennas were used, each would use only one-fourth of the totalantenna elements; etc. There is thus a need for a transmitter having anactive phased array antenna which employs all antenna elements toradiate each directive beam in order to obtain the full directed gain ofwhich the total antenna aperture area is capable, while avoidingexcessive inter-modulation. Further, there is a need for a transmitterhaving an active phased array antenna useful in a situation where noMCPA of reasonable efficiency at the current state of the art can meetthe stringent limits imposed on out-of-band intermodulation radiation,thus necessitating the use of single-carrier amplifiers while stillallowing multiple signals to be radiated by the antenna element array.

The present invention is directed toward overcoming one or more of theabove-mentioned problems.

SUMMARY OF THE INVENTION

An active phased array transmitter is provided by disposing antennaelements on a plane or curved surface along first and second dimensionsangled relative to one another forming an active phased array antenna.Antenna elements in the same row lying along the first dimension arecoupled by a row-associated Butler matrix to provide a number of driveports to which transmit power amplifiers may be connected, withsuccessive drive ports of the Butler matrix corresponding to successive,adjacent beam directions in the plane of the first dimension in which asignal fed to the port will be radiated. Each row of antenna elements isthus capable of radiating a number of fan-shaped beams, with the fan'swide dimension being in a plane perpendicular to the row of antennaelements, while its narrow dimension is in the plane of the antennaelement row.

A first set of transmit power amplifiers is connected to correspondingdrive ports of Butler matrices in different rows, a corresponding driveport being one which radiates the signal supplied to it in a fan beam inthe same plane as the fan beam from a corresponding drive port ofanother row. The set of power amplifiers is thus arranged along thesecond dimension, i.e., the opposite dimension to that of the Butlermatrix connections, and may thus be referred to as a column ofamplifiers. Other sets of amplifiers may be connected to other columnsof corresponding drive ports.

Each column of amplifiers is connected to a corresponding beamformerhaving an input for a signal to be radiated, a set of outputs connectedto the inputs of its respective amplifier column and a phase controlinput to control the relative phasing of the outputs such that the fanbeams radiated by different rows add constructively in only a limitedangular portion within the wide dimension of the fan, thus reducing thefan beams to a spot beam which has a narrow bandwidth in bothdimensions. The direction of the spot beam may be steered in the seconddimension, i.e., perpendicular to the antenna element rows, by means ofthe phase control input, while the spot beam direction along the firstdimension may be selected by routing the signal to be radiated, via arouter, to an appropriate beamformer. Through use of such a transmitter,all of the elements of the phased array antenna are used to form eachradiated beam, while the antenna array simultaneously radiates ndifferent signals in different beam directions using amplifiers thatonly amplify a single signal at a time.

In one implementation, the above-described active phased arraytransmitter is borne on an orbiting satellite for communicatinginformation to and/or between multiple user terminals located atdifferent positions on the earth. Information to be transmitted to agiven user terminal modulates a signal that is routed to the appropriatebeamformer and amplifier column that is able to form a beam anywhere ina fan passing through that user terminal. The control input to thebeamformer is used to select the direction within that fan correspondingto the user terminal's exact location. Simultaneously, other columns ofpower amplifiers may be used to transmit to other user terminals thatare located anywhere within different fans. Thus, simultaneoustransmission to multiple user terminals is permitted so long as they liein different fans. A scheduler or router groups and selects signals orinformation packets for simultaneous transmission such that twotransmissions are never needed in the same fan at the same time.Multiple user terminals lying in the same fan are selected to be servedwith information at sequential times, i.e., using time-sharing or TimeDivision Multiple Access. The grouping may be performed with the aid ofa location-related address contained with the header of data packetsintended for each user terminal.

When the satellite is in a non-geostationary orbit, such as a low earthorbit, it moves rapidly relative to the earth such that the transmittedsignal beams sweep across the earth at several kilometers per second.Thus, the selection of beamformer columns, via the router, and thebeamformer control signals must be altered as a function of time inorder to ensure that data packets transmitted to the same stationaryuser terminal at different times are correctly directed to the samelocation on the earth. This is facilitated by arranging the direction ofelectronic beam steering, effected with the aid of the beamformercontrol signals, to be along or parallel to the satellite's groundtrack, while the direction of selective beam steering, by selection ofthe appropriate beamformer column via the router, to be in a directionat right angles to the satellite's ground track. The beamformers maythen be continuously controlled by means of a timer to compensate fororbital velocity such that, to a first order approximation, spot beamscontinue to be directed to fixed terrestrial locations. Due togeometrical distortions, correcting for satellite movement by routingthe signal to a different beamformer to serve a particular terrestriallocation is still needed, but less often than if the direction ofselective beam steering was in the less favorable direction of theorbital velocity or ground track.

In another implementation, each antenna element is a dual-polarizationelement having two inputs and capable of radiating signals at differentpolarizations simultaneously, for example, horizontal and vertical orleft and right hand circular polarization. One set of Butler matricesconnects rows of elements using their inputs for the first polarization,and another set of Butler matrices connects rows of elements using theirinputs for the second polarization. Corresponding ports of Butlermatrices for the same polarization are connected to a respective columnof power amplifiers and associated beamformers. In this manner, the samearray of antenna elements may be used to create 2n simultaneousdirective transmissions where n is the number of columns of poweramplifiers provided for each polarization, while each amplifier needonly amplify a single signal. The waveform of such signals may be chosento be of constant envelope with the information modulating only thesignal phase, such that efficient class-C amplifiers may be used andoperated at saturation without causing signal distortion.

A feature of the transmitter is the simultaneous transmission ofmultiple signals while employing single-signal amplifiers to reduceunwanted out-of-band radiation due to intermodulation distortion.

Another feature of the transmitter is improved efficiency oftransmission using multiple, directive beams to transmit information touser terminals at different locations on the earth.

Other aspects, objects and advantages can be obtained from a study ofthe application, the drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art transmitter including a single-signal activephased array antenna;

FIG. 2 depicts a prior art transmitter including a multiple-signalactive phased array antenna;

FIG. 3 depicts a prior art transmitter including a dual-polarizationactive phased array antenna for radiating one signal per polarization;

FIG. 4 depicts multiple copies of the prior art transmitter of FIG. 1for multiple signal transmission;

FIG. 5 depicts one embodiment of an active phased array transmitteraccording to the present invention;

FIG. 6 illustrates the preferred orientation of the active phased arraytransmitter on an orbiting satellite;

FIG. 7 depicts another embodiment of an active phased array transmitter;

FIG. 8 depicts yet another embodiment of an active phased arraytransmitter;

FIG. 9 illustrates voltage splitting in a 4×4 port, two-dimensionalButler matrix;

FIGS. 10A–10D illustrate allowed and disallowed spot patterns forsimultaneous illumination with the active phased array transmitter;

FIG. 11 illustrates reuse classes of spots for simultaneous illuminationwith the active phased array transmitter;

FIG. 12 illustrates still another embodiment of an active phased arraytransmitter; and

FIG. 13 depicts a block diagram of a satellite directional transponderincorporating the active phased array transmitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a prior art transmitter, identified generally as 10,for transmitting a single signal. The transmitter 10 includes abeamformer 12, a plurality of power amplifiers 14 and an antenna elementarray 16. The beamformer 12 has a single input for a single signal 18 tobe transmitted, and a beam steering control signal input 20 fordetermining directions of transmission. The beamformer 12 receives theinput signal 18 and generates a number of output signals 22 for drivingthe power amplifiers 14, with each output signal 22 having a phaserelative to the other output signals 22 determined by the beam steeringcontrol input signal 20.

Each power amplifier 14 is connected to a respective one of antennaelements 24 in the antenna element array 16. Each of the antennaelements 24 can alternatively be a sub-array of several elements 24connected in a predetermined manner. By altering the relative phasing ofthe output drive signals 22, via the beamformer control signal 20, thedirection in which the desired signal radiation is reinforced may bevaried, thereby varying the direction of the maximum antenna radiation.In the prior art transmitter 10 shown in FIG. 1, only a single signal 18at a time is amplified by amplifiers 14, such that no intermodulationbetween signals at different frequencies arises. The single signal 18transmitted may be deliberately chosen to be a constant amplitude signalmodulated only in phase with information, such that amplifiers 14 do notneed to be of a linear type, but rather may be an efficient class-Ctype. While not shown in FIG. 1, the radiated spectrum is thenconventionally determined by filtering the phase modulation used toimpress the information on the transmitted signal, and is not distortedor degraded by the amplifiers 14.

FIG. 2 illustrates a prior art phased array transmitter, indicatedgenerally as 30, for transmitting multiple signals. The transmitter 30includes a beamforming matrix 32, a plurality of multi-signal poweramplifiers 34, and the antenna element array 16 as previously describedwith respect to FIG. 1. Beamforming matrix 32 has multiple inputs formultiple signals 36 to be transmitted. The beamforming matrix 32generates output signals 38 which are linear combinations of the inputsignals 36 using a matrix of weighted beam steering coefficients 39 thatdetermines the direction in which maximum radiation will occurindependently for each signal 36. The output linear combinations 38 areamplified by multi-signal power amplifiers 34, which are connected tocorresponding elements 24 of the antenna element array 16. Themulti-signal power amplifiers 34 must be of a linear type thatfaithfully reproduce both a varying amplitude and a varying phase, sincea linear combination of signals has a widely varying amplitude even wheneach signal alone is of constant amplitude.

Departures from ideal linearity in the power amplifiers 34 causeinter-modulation distortion between the different output signals 38,producing unwanted distortion products. The unwanted distortionproducts, known as third order inter-modulation, are at frequencies suchas 2f₁−f₂ or f₁+f₂−f₃, where f₁, f₂ and f₃ are the individual signalcenter frequencies. If the center frequencies are all identical, thenintermodulation occurs on top of the desired signal and on either side.Intermodulation that lies only on top of other desired signals is knownas “in-band intermodulation” and one is free to determine the level ofacceptable in-band intermodulation.

On the other hand, when f₁ lies near one edge of an authorized frequencyband and f₂ lies near the other edge, intermodulation at 2f₁−f₂ and2f₂−f₁ will lie outside the authorized band, and must be suppressedsufficiently to avoid interference with users of the adjacent frequencybands, known as “out-of-band intermodulation”. One is not free todetermine the acceptable level of out-of-band intermodulation withoutreference to other potential users. Various regulatory bodies such asthe FCC in the USA will usually determine the limits for out-of-bandintermodulation based on the envisaged use for the adjacent frequencybands. In some cases, it may be very difficult to meet the regulatoryrequirements utilizing the prior art transmitter 30 shown in FIG. 2without excessive loss of efficiency in the power amplifiers 34.

A conventional technique for meeting more onerous intermodulationsuppression requirements while transmitting more than one signal at atime is through the use of dual-polarization. FIG. 3 illustrates such adual-polarization transmitter, indicated generally as 40. The concept ofdual-polarization is to transmit a first signal using one polarization,for example Right Hand Circular Polarization (RHC) using a first set ofpower amplifiers driven by a first beamformer for directing a firstradiated signal beam, and to transmit a second signal using a secondpolarization, for example a Left Hand Circular Polarization (LHC) withthe aid of a second beamformer and second set of power amplifiers. Itshould be noted that vertical and horizontal polarization may also beimplemented.

The dual-polarization transmitter 40 shown in FIG. 3 includes adual-polarization beamformer 42, a plurality of power amplifier pairs44, and a dual-polarization antenna element array 46.

The dual beamformer 42 includes inputs for two input signals 48 a, 48 bto be radiated simultaneously, and first and second steering controlinputs receiving beam steering control signals 50 a, 50 b for the firstand second polarizations, respectively. The dual beamformer 42 receivesinput signals 48 a, 48 b and generates output drive signals 52 a, 52 bdriving amplifier pairs 44 a, 44 b, with each amplifier 44 a or 44 b ofeach pair receiving only a single signal to be radiated with a givenpolarization. Antenna elements 54 within the dual-polarization antennaelement array 46 may be dual-polarized elements such that the sameantenna array 46 may be used to transmit both polarizationssimultaneously. It should be realized, however, that the above-describedprior art transmitter 40 is limited to simultaneously forming only twobeams, as there are only two independent (orthogonal) polarizationsavailable.

FIG. 4 depicts a prior art transmitter, indicated generally as 60,utilizing multiple single polarization prior art transmitters (as shownin FIG. 1) to transmit more than two signals in different directions atthe same time. The transmitter 60 includes four beamformers 62 a, 62 b,62 c, 62 d which are used to independently steer each of four differentinput signals 64 a, 64 b, 64 c, 64 d via beam steering control signals66 a, 66 b, 66 c, 66 d input to the respective beamformers.

The output drive signals 68 a, 68 b, 68 c, 68 d produced by thebeamformers 62 a, 62 b, 62 c, 62 d drive corresponding sub-arrays (a–d)including single-signal amplifiers and antenna elements in a manner aspreviously described with respect to FIG. 1. These four sub-arrays (a–d)are each a sub-section of the whole array aperture 74, and thus eachbeam is formed using only one-fourth of the total antenna aperture area74, resulting in inefficient use of the available aperture 74.

If each beam had used the whole aperture 74, the beam directivity wouldhave been 6 dB more, that is a beam of four times the peak power,allowing the power amplifiers to be of one-fourth the power level. Thissaving in power is extremely important in orbiting satellites as allpower must be collected by large solar cell arrays. Furthermore, heatdissipation in the vacuum of space is problematic. Although the priorart transmitter 60 of FIG. 4 was described utilizing single polarizationsub-arrays (a–d), dual-polarization arrays, as described with respect toFIG. 3, may be implemented to essentially double the number of signalstransmitted by the transmitter 60.

FIG. 5 depicts a transmitter, indicated generally as 80, fortransmitting multiple independently steerable beams using the wholeantenna array aperture. The transmitter 80 includes an antenna aperturearray 82, columns 84 a–84 n of power amplifiers 85, beamformers 86 a–86n, and a signal router 88. The antenna aperture array 82 includesantenna elements 90 grouped in rows, with each row connected to arespective passive coupler or Butler matrix, and more specifically,matrix 91 for row one, matrix 92 for row two, matrix 93 for row three,matrix 94 for row four, and matrix 95 for row five. The Butler matrices91–95 each have a number of inputs, usually but not necessarily equal tothe number of outputs connected to the antenna elements 90. Each of theButler matrix inputs are indicated with the corresponding Butler matrixreference number followed by a–n, while each of the Butler matrixoutputs are indicated with the corresponding Butler matrix referencenumber followed by a′–n′.

Each input signal to each Butler matrix is split between the outputs ina manner different from, and orthogonal to, the manner in which signalsat the other inputs are split. For instance, input signal 91 a is splitacross outputs 91 a′–91 n′ in a first manner; input 91 b is split acrossoutputs 91 a′–91 n′ in a second manner; etc.

In the simplest form of a two-input, two-output Butler matrix, a signalpresented to the first input maybe split in two, in-phase, half-powercopies at the outputs, while the signal presented at the second input issplit out-of-phase.

A four-input, four-output Butler matrix splits the signals presented atrespective inputs in the phase-sequences:

Output Phases Input 1: 0°  0°  0°  0° Input 2: 0°  90° 180° 270° Input3: 0° 180° 360° 540° Input 4: 0° 270° 540° 810°The signals to successive inputs split with a phase incrementedsuccessively through multiples of an incremental phase; in the aboveexample the phase increments are a multiple of 90°. The incrementalphase shift being 0° for Input 1, 90° for Input 2, 180° for Input 3, and270° for Input 4.

Referring again to FIG. 5, the Butler matrices 91–95 are eight-input,eight-output Butler matrices which accordingly use phase incrementswhich are a multiple of 45°. More specifically, the Butler matrices91–95 split the signals presented at respective inputs in the followingphase-sequences:

Output Phases Input 1: 0° 0° 0° 0° 0° 0° 0° 0° Input 2: 0° 45° 90° 135°180° 225° 270° 315° Input 3: 0° 90° 180° 270° 360° 450° 540° 630° Input4: 0° 135° 270° 405° 540° 675° 810° 945° Input 5: 0° 180° 360° 540° 720°900° 1080° 1260° Input 6: 0° 225° 450° 675° 900° 1125° 1350° 1575° Input7: 0° 270° 540° 810° 1080° 1350° 1620° 1890° Input 8: 0° 315° 630° 945°1260° 1575° 1890° 2205°While an eight-input, eight-output Butler matrix is shown in FIG. 5, itshould be understood that Butler matrices of any order can be utilizedwithout departing from the spirit and scope of the present invention.Butler matrices can be produced by suitable stripline or waveguidestructures. Waveguide Butler matrices may be constructed economically byinjection molding of plastic, which is subsequently metallized to formthe conducting waveguide cavities.

Each signal input to Butler matrix inputs 91 a–91 n, 92 a–92 n, etc., istransmitted by the associated row of antenna elements 90 connected toits outputs 91 a′–91 n′, 92 a′–92 n′, etc. with a fan-shaped radiationbeam 1-n as shown in FIG. 6. Different Butler matrix inputs produce fanbeams at different angles as illustrated by fan 1, fan 2, . . . fan n ofFIG. 6, which will be described in greater detail below.

Referring back to FIG. 5, a first column 84 a of amplifiers 85 isconnected to the first inputs 91 a–95 a of each Butler matrix 91–95,that is to the inputs which produce the same fan angle from every row.When all the amplifiers 85 are excited with a signal in the appropriaterelative phasing, radiation from all the rows 1–5 will addconstructively in only one direction within fan 1 (see FIG. 6), thuscreating a spot beam. The location of the spot beam within fan 1 (seeFIG. 6) may be controlled by varying the beam steering control signal 98a input to beamformer 86 a.

Similarly, there are columns 84 b–84 n of amplifiers 85, with associatedbeamformers 86 b–86 n, connected to the other Butler matrix inputsnamely, inputs b, c, . . . n, which are not fully shown in FIG. 5 topreserve clarity. More specifically, the number of amplifier columns 84a–84 n and beamformers 86 a–86 n depends on the number of inputs to theButler matrices 91–95. Each beamformer 86 a–86 n controls the directionof the spot beam within a different fan, thereby giving n independentlysteerable beams. Each beam is formed using all of the elements 90 of theantenna aperture array 82, while each power amplifier column 84 a–84 namplifies only a single signal at a time, thus avoiding intermodulation.

It should be noted that the transmitter 80 can be configured without theplurality of amplifier columns 84 a–84 n, but with n power amplifiers(not shown), with each power amplifier amplifying a different signal 99a–99 n input to respective beamformers 86 a–86 n. In this case, however,beamformers 86 a–86 n will have to operate on high power, amplifiedsignal which is not preferred.

In operation, the signal router 88 receives the various signals 1-nwhich are to be transmitted simultaneously. To direct radiation of asignal to a given spot, the signal router 88 first determines the fan inwhich the spot lies, and routes that signal 99 a–99 n to the beamformer86 a–86 n for that fan. The selected beamformer 86 a–86 n iselectronically steered, via steering control signals 98 a–98 n, toeffect radiation in the desired spot direction within the fan for thatsignal 99 a–99 n. The signal router 88 may thus direct radiation of onesignal in each fan at a time.

Beamformers 86 a–86 n provide for continuous variation of the spotdirection within the associated fan, while the router 88 allowsselection of a fan. Thus, the transmitter 80 of FIG. 5 providescontinuous beam steering in one plane with switched beam steering in theother plane.

FIG. 6 illustrates the preferred orientation of the plane of continuous,electronic steering versus the plane of switched, discrete-step steeringwhen the transmitter 80 is used on a low earth orbiting satellite 100.The long dimension of the fans 1-n, shown by arrow 102, along whichcontinuous steering is effected via the beamformers 86 a–86 n ispreferably oriented along the ground track of the satellite 100; thatis, in the direction of the orbital motion or velocity of the satellite100. As shown in FIG. 6, different fans occupy different positions tothe right and left of the ground track, with coverage by the two extremefans 1 and n illustrated in FIG. 6. The switched, discrete-step steeringbetween fans occurs in the plane or dimension generally depicted byarrow 104 and is effected by the router 88.

A spot beam directed toward a particular user terminal, as shown in thecoverage region of fan 1, may then be maintained by controlling theappropriate beamformer 86 a to move the beam progressively backwards inthe opposite direction to the satellite's 100 motion, thus compensatingfor satellite movement. Due to geometrical distortions, namely that theextremes of the fan beams are farther away from the ground track thanthe center of the fans, it may eventually become necessary for therouter 88 to switch the fan used to serve a particular user terminal.However, this switching is at a much reduced rate than if the fans hadbeen oriented to cover regions elongated across the ground track.

In cases where the number of antenna array elements 90 in a row islarge, for example greater than about 16, a Butler matrix to couple allelements in a row having sixteen inputs and sixteen outputs may becomeexcessively large as a waveguide structure or else excessively lossy.When such a full-sized Butler matrix is used for the Butler matrices91–95 in FIG. 5, e.g., a 16×16 Butler matrix, the number of fan-shapedsub-regions produced is sixteen and the number of beamformers 86 a–86 nand columns 84 a–84 n of independent amplifiers 85 which can beconnected is also sixteen, providing the ability to radiate sixteendifferent signals, one per fan, using only single signal amplifiers.However, as shown in FIG. 7, the transmitter can be usefully configuredto generate fewer than the maximum number of simultaneous beams.

FIG. 7 depicts a transmitter 110 wherein the row of Butler matrices91–95 of FIG. 5 have been split into smaller Butler matrices. Morespecifically, the 8×8 Butler matrix 91 in FIG. 5 has been replaced withtwo 4×4 Butler matrices 112 a, 112 a′ in FIG. 7; the 8×8 Butler matrix92 in FIG. 5 has been replaced by two 4×4 Butler matrices 114 a, 114 a′in FIG. 7; etc. Similarly, in a transmitter utilizing a full size 16×16Butler matrix, four 4×4 Butler matrices can be replaced therefore.Corresponding input ports of each Butler matrix 112 a, 112 a′–120 a, 120a′ are driven by matrices 122 a–122 n of amplifiers 85, with each matrixhaving a column of amplifiers 85 for each of the undersized Butlermatrices 112 a, 112 a′–120 a, 120 a′. For clarity purposes, onlybeamformer 86 a and amplifier matrix 122 a are shown in FIG. 7.

Thus, in the transmitter 110 shown in FIG. 7, there are four amplifiermatrices 122 a–122 n, each having two columns. In the case of sixteenrow elements coupled by four, 4×4 Butler matrices, there would be fouramplifier matrices, each having four columns of amplifiers, making amatrix of 4×M amplifiers, where M is the number of rows, i.e., antennaelements 90 in a column.

All amplifiers 85 of same matrix connect to corresponding inputs of allButler matrices, i.e., to inputs number 1. Other, independent amplifiermatrices may be connected to the other Butler matrix inputs number 2, 3,etc. Thus, when using 4×4 Butler matrices, four independent amplifiermatrices 122 a–122 n may be used, allowing four independent beams to begenerated using single-signal amplifiers. The use of sub-sized Butlermatrices effectively produces fewer, broader fans. Beamformers 86 a–86n, steering control signals 98 a–98 n and signal router 88 function aspreviously described with respect to FIG. 5, albeit there need now beonly half as many beamformers 86 a–86 n and steering control signals 98a–98 n.

The beamformers 86 a–86 n of FIG. 7 produce signals controlled inrelative phasing down each column to produce beams steered along the fanregion's largest dimension as before (see arrow 102 in FIG. 6), but nowalso controls the relative phasing between the different columns ofamplifiers within the same amplifier matrix 122 a–122 n, so as toproduce fine beam steering across the fan's narrow dimension in thedirection of arrow 104 in FIG. 6.

Thus, in operation, the signal router 88 receives signals 1-n to betransmitted and determines the coarse fan-shaped region in which theuser to which the signal is to be transmitted lies. The router 88selects the beamformer 86 a–86 n and associated power amplifier matrix122 a–122 n that are configured to transmit within that particularregion. The beamformer 86 a–86 n is controlled, via steering controlsignals 98 a–98 n, to fine-steer the beam both along and across theselected region to place the peak beam directivity as close as possibleto the user's location.

In FIGS. 5 and 7, the Butler matrices are connected only to elements inthe same row. The Butler matrices function as partial beamformers,forming the directivity only in one plane in FIG. 5, and forming onlypart of the total directivity in one plane in FIG. 7. The beamformers 86a–86 n complete the formation of the required directivity by forming allof the directivity in the column plane (FIG. 5), and in the case of FIG.7 completing the formation of the directivity in the row plane.

A further embodiment of the transmitter is shown in FIG. 8. Transmitter130 depicted in FIG. 8 includes two-dimensional Butler matrices 132a–132 d which connect both row and column elements 90 of the antennaaperture array 82. FIG. 8 depicts four 2×2 two-dimensional Butlermatrices 132 a–132 d, however, other sized matrices can be utilizedwithout departing from the spirit and scope of the present invention.The two-dimensional Butler matrices form part of the directivity in boththe row and column planes.

Each input 1, 2, 3, 4 to the Butler matrices 132 a–132 d include aseparate power amplifier array 134 a–134 d including a separatesingle-signal power amplifier 85 for each Butler matrix 132 a–132 d.Similar to FIGS. 5 and 7, there are separate beamformers 86 a–86 d withaccompanying steering control signals 98 a–98 d for each of the fourinputs of the Butler matrices 132 a–132 d. For clarity purposes, onlybeamformer 86 a and associated amplifier matrix 134 a are depicted inFIG. 8. Thus, in FIG. 8 there are four beamformers 86 a–86 d, foursteering controls signals 98 a–98 d, and four power amplifier arrays 134a–134 d. Accordingly, the transmitter 130 in FIG. 8 is capable oftransmitting four separate signals 99 a–99 d simultaneously.

The two-dimensional Butler matrices 132 a–132 d effectively perform ananalog Discrete Fourier Transform (DFT) using the Fast Fourier Transform(FFT) structure. Two-dimensional FFT structures are simpler thanone-dimensional FFT structures of the same number of input and outputsignals, and thus the configuration of the two-dimensional Butlermatrices 132 a–132 d of FIG. 8 can be advantageous. For example, a fourinput, four output two-dimensional Butler matrix can be made using only180° hybrid junctions or “Magic Tee's” in waveguide terminology.Alternatively, as shown in FIG. 9, 90° couplers, such as branch linecouplers, can be used.

In FIG. 9, four signal voltages a, b, c, d are input on the left and aresplit in different combinations to form the four outputs at the right. Ascaling of two at the input and root-2 after the first two couplers 140,142 reflects conservation of energy. The four outputs from couplers 144,146 drive a square sub-array of four antenna elements 90 in the wholearray 82 of FIG. 8, such that the signals a, b, c, d are radiatedrespectively into each of the four quadrants of the coverage area,which, in the case of an orbiting satellite might be described as: (a)to the right of the ground track and behind the sub-satellite point; (b)to the left and behind; (c) to the right and forward of thesub-satellite point; and (d) to the left and forward.

Whichever configuration is used, the principle for directing a signal isthe same as previously outlined, namely, first determining the coarseregion (which are formed by the Butler matrices) in which the user lies,selecting the corresponding power amplifier, array and beamformer, thenadjusting the beamformer to fine steer the beam to the user. Withinother design constraints such as Butler matrix complexity and loss,preference should be given to forming coarse regions having a greaterdimension along the direction of orbital motion (see arrow 102 in FIG.6) with the narrower dimension at right angles to the satellite motion(see arrow 104 in FIG. 6), thus allowing the electronic beamformers 86a–86 n to be preprogrammed to compensate for satellite motion.

In one application, the radiated signal is a wideband digitally phasedmodulated signal carrying bursts of high bit rate information using allof an allocated bandwidth within the radio communications spectrum. Asmany such signals may be radiated simultaneously as there arebeamformers, that is n times, so that the entire allocated bandwidth isreused n times. When a given part of the spectrum is reused, thedistance between spots in which the same spectrum is used must besufficient to avoid self-interference, which is also co-channelinterference. This is avoided if the frequency channel is reused in asecond beam which is well enough away from the main lobe of a firstbeam. FIGS. 10A–10D show allowed and disallowed patterns of frequencyreuse.

In FIG. 10A, the shaded spots indicate allowed locations of simultaneoususe, as there is always at least a separation of one or more spot beamdiameters between the spots in use. FIG. 10B illustrates that it isperfectly acceptable to illuminate spots that have the same coordinates,i.e., d and f, within their respective fans, providing that this doesnot occur in adjacent fans.

On the other hand, FIG. 10C illustrates a disallowed pattern due tohaving two spots (1 c and 1 f) in the same fan (fan 1) simultaneously.Such a spot beam pattern is not capable when single-signal poweramplifiers are used to create each fan.

FIG. 10D shows another disallowed pattern in which spots having the samedirection (c) within their respective fans occur in adjacent fans (fan 1and fan 2). Such a separation is insufficient to avoid co-channelinterference. Further, even the spot beams that are diagonally adjacentat locations 3 f and 4 e may be too closely spaced to avoid co-channelinterference. Accordingly, the signal router 88 must queue signals fortransmission to different terminals according to their locations in sucha way as to utilize the inventive transmitters capability to transmit nsimultaneous beams, but in such a way as to avoid violation of reusedistance criteria.

An alternative method to impose reuse distance criteria is illustratedin FIG. 11. In FIG. 11, the vertical columns of circles or spots alongfans 1–6 are shown offset between adjacent columns, as is customary whendiscussing reuse in cellular radio systems. Reuse of a frequency channelmay occur in regularly spaced places forming a reuse pattern. Differentchannels must be used in all other places.

FIG. 11 illustrates distributing the use of three frequency channels orthree timeslots numbered 1, 2, 3 in a so-called three-cell patternacross six fans. The separation between cells, or spot beams, such asthe shaded cells corresponding to channel 1, is equal to the square rootof the cell pattern size (3 in this example for the three frequencychannels) multiplied by the cell diameter. Thus, a larger cell patternincreases the separation between co-channel cells, reducing their mutualinterferences. In the example of FIG. 11, it is assumed that a reusepattern size of 3 reduces mutual interference to an acceptable level,and that the same channel may thus be used in all of the shaded spotssimultaneously.

The inventive transmitter is, however, only capable of illuminating onespot in each vertical column or fan at a time, due to the use of singlefrequency power amplifiers for each fan beam. The selected spot may,however, be any of the shaded spots in the column at a given first timeinstant. At a second time instant, anyone of the spots containing a twomay be selected in each column, and at a third instant any one of thespots containing a three may be selected in each column. Thus, usingthis strategy, any one of a first ⅓ of the spots in each column isilluminated in a first timeslot of a repetitive Time Division MultipleAccess (TDMA) frame period; any one of a second ⅓ of the spots in eachcolumn may be selected during a second timeslot; and any one of a third⅓ of the spots in each column may be selected in a third timeslot. Atany instant, which of the allowed spots in a column, or fan, that isactually selected may depend for example on the backlog of traffic forthat particular spot. If no traffic exists for a spot, it is of coursenot a candidate to be selected. If a data packet destined for a user ina particular spot has been waiting in a queue for a period of timelonger than data packets destined for other spots belonging to the sametimeslot reuse group, then that spot would be preferentially selectedwithin the column for illumination at the earliest opportunity, i.e.,upon the next occurrence of its timeslot.

The above-described regular reuse pattern effectively creates threemajor queues, and each queue receives one out of three timeslots,exactly ⅓ of the transmit capacity, in which to transmit traffic whetherneeded or not. Within each queue, each column or fan of cells disposesof the capacity of one transmit beam, whether needed or not.Occasionally, this could lead to inefficiency where the transmitcapacity available for one column of cells was not used while thetraffic demands in another column of cells temporarily exceeded thecapacity available for that column. This so-called trunking loss is thepenalty one must pay for dividing up transmission into three timeslotsof fixed size.

Other reuse patterns can be used when such inefficiency is encounteredand cannot be tolerated. For example, a larger number of timeslots thatare a multiple of the reuse pattern size can be defined, such as twelvetimeslots (4×3) in the present example of a three-cell pattern. Eachreuse group may be visited up to four timeslots out of twelve in thecase of equal traffic demand per group. However, if there is one cellthat demands the largest amount of traffic flow, then the reuse groupcontaining that cell may be allocated more than four out of twelvetimeslots by reducing the number of timeslots to less than four foranother reuse group. This is equivalent to varying the original timeslotsizes away from exactly ⅓ of the frame in discrete steps of 1/12 of theframe period to better adapt to the traffic pattern.

Another exemplary use pattern is to transmit, during the next fractionof a frame period, to a spot for which a data packet has been queuedlongest. That spot is located in a particular row in a particularcolumn, and referring to FIG. 11, we may not then transmitsimultaneously in either of the adjacent spots in adjacent rows in thatcolumn. All other spots in the adjacent rows of that column are howevercandidates for receiving a signal. Selection among the candidates in theadjacent rows is based on the length of time packets have been waitingfor delivery in those spots. Having selected one spot in each of theadjacent rows, this disallows two spots in each of the next-adjacentrows from being served at the same time. All others are, however,possible candidates for selection and are selected once more upon packetdelivery backlog. This method continues until one spot in each row of acolumn has been selected to receive service at the next timeslot. Whilethese packets are being delivered, the scheduler, which may form part ofthe router 88, determines the spots that shall receive service in thefollowing timeslot. This method is not constrained to a fixed reuseplan, and therefore offers more flexibility in servicing a very unequaltraffic demand for different cells.

The inventive transmitter may also be constructed using bothpolarizations to transmit twice as many simultaneous beams from the sameantenna aperture. Referring to FIG. 12, such a dual-polarizationtransmitter, indicated generally as 150, is depicted. The signal router88 receives the incoming signals 1-2 n and routes respective signalpairs 152 a, 152 a′–152 n, 152 n′ to corresponding dual beamformers 154a–152 n. The dual beamformers 154 a–154 n each include first and secondsteering control inputs receiving beam steering control signals 156 a,156 a′–156 n, 156 n′ for the first and second polarizations,respectively. The dual beamformers 154 a–154 n receive input signals 152a, 152 a′–152 n, 152 n, and generate corresponding pairs of output drivesignals 158 a, 158 a′–158 n, 158 n′ to columns 160 a–160 n of amplifierpairs 85 a, 85 a′. For clarity purposes, only dual beamformer 154 a andcolumn 160 a of amplifier pairs 85 a, 85 a′ is depicted in FIG. 12.

While operation of transmitter 150 will be described with respect todual beamformer 154 a and amplifier column 160 a, it should beunderstood that the other beamformers 154 b–154 n and associatedamplifier columns 160 b–160 n operate in the same manner. The dualbeamformer 154 a applies the output drive signal pairs 158 a, 158 a′ tocorresponding amplifier pairs 85 a, 85 a′ in amplifier column 160 a,which in turn supply the signals to a corresponding input of Butlermatrix pairs 162 a, 162 a′–166 a, 166 a′.

As in the prior art of FIG. 3, dual-polarization antenna elements 54 aredistributed over a two-dimensional antenna aperture 46. Each element 54has a connection for transmitting an RHC polarized signal and a separateconnection for transmitting an LHC polarized signal, however, verticaland horizontal polarizations may also be used. All of the RHCconnections for a row of elements 54 are then connected to the outputsof an RHC Butler matrix 162 a, while all the LHC connections for a rowof elements 54 are connected to a similar LHC Butler matrix 162 a′.Other Butler matrix pairs are connected to the antenna elements 54 in asimilar manner.

The transmitter 150 operates in the same manner as previously describedwith respect to the transmitter 80 of FIG. 5, except that now twice asmany signals may be transmitted utilizing both RHC and LHC elementconnections. Further, similar to FIG. 5, the number of amplifier paircolumns 160 a–160 n and dual beamformers 154 a–154 n are equal to thenumber of Butler matrix input pairs.

Assuming sufficient polarization isolation can be achieved to allow thesame frequency to be used twice in the same location at the same timewith opposite polarizations, then the reuse pattern of FIG. 11 can beused for both polarizations. In this case, each spot may be illuminatedwith either RHC or LHC polarization, or both, at the same time. Thisimplies that user terminals in every location are somehow divided intothose receiving service via RHC polarization and those receiving servicevia LHC polarization. Preferably, the polarization can be fixed for eachterminal to avoid the extra complexity of being able to receivepolarizations.

When very high frequencies such as 20–30 GHz are used for communication,polarization corruption can take place due to the presence ofnon-spherical water droplets in the atmosphere, thus reducing the amountof polarization isolation. In that case, it may be insufficient to relyentirely upon polarization isolation, and instead utilizing partlypolarization isolation and partly spacial isolation may be necessary.With reference to FIG. 11, assuming that the shaded spots are beingilluminated by RHC polarization during timeslot 1, the intervening spotslabelled “2” may be receiving service using LHC polarization. That is,the spots which are illuminated by RHC in timeslot 2 may be illuminatedby LHC in timeslot 1. Likewise, the spots which are illuminated by RHCin timeslot 3 may be illuminated by LHC in timeslot 2, and the spotswhich were illuminated by RHC in timeslot 1 may be eliminated by LHC intimeslot 3. Since the two polarizations are never used in the same placeat the same time, this pattern does not rely entirely upon polarizationisolation to prevent co-channel interference. Both RHC and LHCpolarizations may also be used without the constraint of a regular reusepattern such as shown in FIG. 11.

As previously described, a scheduler (170 in FIG. 13) determines thespot that has the greatest traffic backlog to receive priority treatmentduring the next available timeslot. This principle can be extended todetermining which spot and polarization has the greatest backlog in thecase where user terminals are only configured for one polarization at atime and thus must be served with a specific polarization. The scheduler170 thus determines which of the two polarizations will be used first toserve the chosen spot in the next timeslot. The scheduler 170 thencontinues as before to determine one spot in each of the other rows,excluding the use of adjacent spots in adjacent rows, that can receiveservice at the same time using that same polarization. Independently,the scheduler 170 can determine the spot which has the greatest backlogfor service using the opposite polarization, excluding the spots alreadydetermined to receive service using the first polarization in order toavoid using both polarizations in the same place at the same time.

Efficiency may result in determining the spots that shall receive RHCservice and the spots that shall simultaneously receive LHC service inan interleaved manner, instead of determining all RHC spots firstfollowed by all LHC spots. First, the spot and polarization having thegreatest backlog is determined. Then, the spot having the greatestbacklog for service with the opposite polarization is determined,excluding the first spot if it is desired to avoid use of bothpolarizations in the same spot at the same time. Next, the spot havingthe next greatest backlog is determined, excluding the alreadydetermined spots and spots adjacent to those that use the samepolarization, but not excluding adjacent spots that use the oppositepolarization, and excluding columns containing an already determinedspot needing the same polarization, as only one beam per column perpolarization can be created. It should be understood that many othervariations of scheduler strategies can be implemented by a person ofordinary skill in the art without departing from the spirit and scope ofthe present invention, and it is not an objective herein to provide anexhaustive list of all possibilities based on the principle ofscheduling traffic based on demand while avoiding use of bothpolarizations in the same place at the same time.

In yet another implementation, the polarization used to serve aparticular spot or region on the earth is fixed to be either RHC or LHC.User terminals sold and installed in a particular region would then beequipped to receive only the designated polarization for that region.The entire surface of the earth would be mapped to LHC and RHC regionsin an interleaved manner. For example, alternate columns or fans ofspots in FIGS. 10A–10D could be designated as LHC and RHC respectively.Then the pattern of service which was disallowed in FIG. 10D would nolonger be disallowed, as adjacent spots in different columns wouldemploy opposite polarizations. This allows for a simpler scheduleralgorithm since the spot to serve during a particular timeslot can bedecided independently for each column of cells without reference to thechoice in other columns of cells. The algorithm thus reduces todetermining the cell in each column having the greatest traffic backlog.

Other regular frequency or timeslot reuse patterns exist, of a sizegiven by integers of the form i²+j²−ij. For example, with i=1, j=2 thisexpression evaluates to 3, confirming that a regular 3–cell patternexists. With i=j=2, a regular 4–cell pattern exists. Other regular cellpattern sizes are 7, 9, 12, 13, 16, etc.

As is generally known in the art, the size of a cell used in a satellitecommunication system can advantageously be much smaller than themain-lobe beam diameter produced by the phased arrayantenna/transmitter. A cell can be defined to lie only around the gainpeak of the antenna beam and not down to the −4 dB edge, as waspreviously customary. However, the spacing of cells using the samechannel must be chosen with regard to the antenna's beamwidth and thusits ability to discriminate radiation coming from different angles. Bydefining small cells that occupy only the beam peak, the co-channel cellspacing can be reduced as user terminals will not now lie at worst casebeam edge locations. The co-channel cell spacing can then be reduced toapproximately two times the beam radius, where beam radius is measuredat the −4.5 dB point relative to beam peak, when a cell occupies thearea only out to ¼ the cell radius from beam center. Since a cell isthen only ¼ of the area of the beam's −4.5 dB coverage, a 16–cellpattern is required to cover the whole surface area with different cellswithin the 16–cell cluster not employing the same frequency resource atthe same time. Since reuse distance, i.e., the distance between cellsusing the same frequency resource at the same time, is reduced by thistechnique, the system capacity defined in traffic per square kilometeris increased. Thus, the use of larger or smaller reuse patterns does notdetermine system capacity for multi-beam satellite communicationsystems, but rather the reuse distance. The reuse distance is minimized,thus maximizing system capacity when a reuse pattern size is large andthe cell size is small, as is generally known in the art.

When, in addition to frequency and time, polarization is introduced at aresource, there is an interest in defining a resource reuse pattern thatincludes frequency, time and polarization. Since there are only twoorthogonal polarizations, the product of the number of frequencies,timeslots and polarizations available will always be even, thussuggesting the use of one of the even-sized reuse patterns such as 4, 12or 16. For example, a system having two timeslots combined with twopolarizations can be employed instead of the 3-timeslot×2-polarizationsystem previously discussed with respect to FIG. 11. Thus, a four-cellreuse pattern could be created by assigning one of the four resources(t1, RHC), (t1, LHC), (t2,RHC), (t2,LHC) to cells in a fixed manner. Forexample, (t1,RHC) can be alternated with (t2,RHC) up the first column orfan of FIG. 11, thus making the entire column RHC. The second column ofcells may be assigned to use (t1,LHC) alternating with (t2,LHC), thusmaking the second column or fan entirely LHC, and so on alternating thepolarization used between successive columns or fans of cells. In thismanner, not only is the polarization fixed for a user terminal in afixed location, but also the ½ of the frame period in which it shallreceive is also fixed, thus reducing its receiving processing andallowing it to transmit in the remaining half period. This may avoidsimultaneous transmit and receive, which can be problematic for low costterminals. With a larger reuse pattern, such as an8-timeslot×2–polarization (a 16-cell pattern), each receiver onlyreceives a single polarization for ⅛ of the time, further reducingcomplexity in processing.

When a number of beams that a satellite can simultaneously create islimited due to use of single-signal power amplifiers for intermodulationreasons, each beam, when created, should transmit with the maximum datarate possible, that is, the single carrier signal in the beam should bemodulated with the highest information rate possible that fits withinthe total allocated band. Power limitations may prevent the maximum datarate from being used, in which case the frequency band can be dividedinto a number of lower information rate channels which can be combinedwith timeslots and polarizations in a joint reuse plan. The inventivemulti-beam transmitter allows different beams to transmit differentfrequencies, and allows transmission of two frequencies even in the samefan using different polarizations.

Referring now to FIG. 13, a block diagram of a satellite communicationstransponder, indicated generally as 172, embodying the inventivemulti-beam transmitter is depicted. Signals 174 are received fromterrestrial terminals or other satellites (not shown) by a multi-beamreceive antenna 176. Unlike the antenna used for transmission, thereceive antenna 176 is not restricted to receive single frequencychannels at a time because the low level received signals do notchallenge the linearity of the receive amplifier components. U.S. Pat.Nos. 5,539,730 and 5,566,168 to applicant, which are herein incorporatedby reference, disclose that different access protocols may therefore bein order for the uplink as compared to the downlink. For example,multiple narrow frequency channels combined with few, long timeslots(narrowband TDMA) can be advantageous for the uplink by reducing theuser terminal peak transmit power requirement. Wideband TDMA having fewor only one frequency channel combined with many, short timeslots ishowever beneficial for the downlink as it allows efficient class-Camplifiers to be used without causing intermodulation.

When different uplink and downlink formats are used, and directterminal-to-terminal communication via the satellite relay is employed,then the satellite must perform format conversion on board. Onesimplified method of converting narrowband TDMA to wideband TDMA isdescribed in U.S. patent application Ser. No. 08/581,110, filed Dec. 29,1995 to applicant, which is herein incorporated by reference. The methodof the above-identified application includes sampling narrowbandreceived signals at a rate at least equal to the Nyquist rate, storingthe samples temporarily in a buffer during an uplink timeslot, thenbursting out the buffer contents at a higher rate during a shorterdownlink slot, thereby increasing the downlink TDMA bandwidth by a givenfactor. Access control is provided in the above-identified applicationby a central ground station that orchestrates the creation of cipherkeys in common between the two communicating terminals, which areenvisaged to be of telephone type that operate mainly in one-on-onecommunications. For Internet terminals, however, the terminal with whicha given terminal wishes to communicate data to or from is notnecessarily fixed for a given session. It is therefore impractical toestablish a session key for each possible pairing of terminals. On theother hand, if terminals use different session keys, then decryption andre-encryption of traffic must be performed on board the satellite. Thissuggests that the satellite transponder 172 will need to be of thedemodulate-remodulate type.

Consequently, block 178 in FIG. 11 provides multi-channel downconversion, demodulation and error correction decoding for all uplinktraffic. The output of block 178 is in the form of binary bits or datasymbols that are fed into an access control unit 180. Access controlunit 180 may check that a Cyclic Redundancy Check (CRC) code, which is afunction of all data bits, has been decoded properly, and reject anytraffic packets with uncorrected errors. The CRC code and other fieldsof the traffic bursts may in fact be enciphered using a session keyestablished for the originating terminal during an initial logonprocedure. The logon procedure can involve communication between acentral network control computer located on the satellite or on theground, during which the network can issue a random challenge andreceive an authentication response from the terminal to authenticate itas a genuine subscriber, a byproduct of this process being the creationof a temporary session key. A temporary abbreviated terminal identitycode may also be issued for the session to reduce overhead in the packetdata headers. Conventional authentication processes are described inU.S. Pat. Nos. 5,091,942, 5,282,250, 5,390,245 and 5,559,886, all ofwhich are herein incorporated by reference.

Thus, access control block 180 can include verification that the packetwas transmitted by a authenticated subscriber who can be billed for thecommunications service provided. If a packet contains decoding errors orcannot be verified to have been transmitted by an authentic user, itwill not be relayed by the satellite and will not consume any capacityresources. A method for verifying authenticity needs to be designedtaking into account all known methods of fraud, such as authenticationwith the unwitting help of a known terminal, eavesdropping on authentictransmissions to determine a valid code, etc.

The most secure method to prevent fraud is to encipher data packetheaders and payload together using a block cipher such as DES prior toerror-correction coding, such that the transmitted signature is notbased on just the session key and the terminal ID, which do not change,but also on the data content or payload, which does change. Thus, theonly way to cause the satellite to relay a false packet would be torepeat the entire packet, which has no utility to fraudster. Moreover,this could, if necessary, be prevented by including a real time clockidentification in each packet to establish a limited window of validity.The same packet transmitted later with an out-of-date clock value wouldthus be rejected. The real time clock value can, for example, be a framecounter value that counts TDMA frame periods of the system, as isalready known to be useful for enciphering from U.S. Pat. Nos. 5,148,485and 5,060,266 to applicant.

At block 182, the destination terminal ID's contained in the packetheaders are related to the position or cell in which the destinationterminal is located. In principle, this information could already beconverted to beam parameters for downlink transmission, however, packetsare not necessarily transmitted instantaneously, but are queued inbuffer memory 184 for a short time, during which the satellite may havemoved. Consequently, the derivation of beam steering parameters by therouter occurs at block 186 immediately prior to transmission.

As previously discussed, a preferred concept of routing and schedulingfor the downlink is to maintain separate queues for data packetsdestined for each cell. Scheduler 170 receives packets from the queuebuffer memory 184 using any of the above-discussed strategies. Forexample, if a subset of cells is to be visited by the downlink beamsduring the next timeslot, then the scheduler 170 only considers packetsdestined for cells within that subset. After selecting the packets forsimultaneous transmission, the exact beam steering parameters for eachbeam are determined by the router at block 186.

More specifically, the scheduler 170 includes a backlog tracker 200determining a waiting time for each data packet, the waiting timeindicative of how long each data packet has waited in the buffer memory184 for transmission. A first selector 202 selects the data packet thathas been waiting the longest, as indicated by the backlog tracker 200. Asecond selector 204 further selects data packets for simultaneoustransmission in descending order of waiting time. The second selector204 skips data packets requiring a transmit direction incompatible withthe direction of transmission of a data packet previously selected fortransmission at the same time. For example, a transmit direction may bedeemed incompatible by the second selector 204 if: 1) it can only becreated using a directional transmission beam that will be fullyoccupied in transmitting an already selected data packet; or 2) it liestoo close in direction to the direction of transmission of a data packetalready selected for transmission at the same time. A direction tooclose in direction to another direction may be one in which an angularseparation of the directions is less than a minimum value sufficient toavoid mutual interference. Such minimum angular separation may include afirst separation when the polarizations of the transmission used in eachdirection are the same, and a second separation when the polarizationsof transmission used in each direction are different.

In order for the scheduler 170 to work properly, the scheduler 170 mustfirst determine a subset of directions in which the directed beams canbe transmitted in a next transmission. Then, by mapping a destinationidentifier code stored with each data packet to a corresponding beamdirection, the scheduler 170, via first and second selectors 202, 204,selects only data packets whose destinations map to a direction withinthe subset of directions. The first and second selectors 202, 204 orderthe selected data packets according to how long they have been stored inthe buffer memory 184, with the selected data packets which have beenstored the longest ordered first.

An indication flag may be set in the buffer memory 184 in associationwith the selected data packets already transmitted, and/or the selecteddata packets which have been transmitted may be deleted from the buffermemory 184, releasing memory locations in the buffer memory 184 so as toprovide storage capacity for new data packets.

Block 186 may contain routing information for routing a signal to aparticular fan. The routing information may be updated from the groundnetwork control computer at a relatively slow rate, while signals forsteering the spot beam within a selected fan may be generated in realtime with the aid of an on-board clock and systematic update proceduresthat compensate for satellite movement. Some interaction between packetselection from buffer memory 184 and determination of the fan in whichthe destination lies may be required for the scheduler 170 to functioncorrectly, due to the fact that only one beam of each polarization maybe created per fan, combined with the fact that fan coverage areas onthe ground do not comprise straight lines or columns of cells, but arerather curved due to geometrical distortion. Therefore, it may belogical to perform the determination of which fan each packet will betransmitted to at block 182, which is possible due to the slower speedat which this parameter changes due to satellite movement when thepreferred array orientation is employed, i.e., direction of electronicsteering along the satellite's ground track with direction of switchedfan selection across the ground track.

The beam steering parameters for each beam are determined for the packetto be transmitted in the selected fan and communicated to the beamformerdealing with that fan and polarization. The beamformers, poweramplifiers, Butler matrices and dual polarization antenna elements ofthe various embodiments previously described may be included withintransmitter block 188 of FIG. 10. Block 190 comprises downlink burstformatting, error correction coding, modulation and upconversion to theallocated downlink frequency band for transmission. Downlink formattingcan comprise multiplexing the destination ID's for all packets destinedfor terminals in the same cell separately from the payloads destined forthose terminals, such that user terminals do not have to continuallydemodulate data at the generally high transmission bit rate and furtheromit decoding of non-intended messages sent to the terminal.

While the invention has been described with particular reference to thedrawings, it should be understood that various modifications could bemade without departing from the spirit and scope of the presentinvention.

1. A scheduler for selecting data packets from a buffer memory to betransmitted in a next time period using multiple directionaltransmission beams controllable in direction, said scheduler comprising:a backlog tracker determining a waiting time for each data packet, saidwaiting time indicative of how long each data packet has waited in thebuffer memory for transmission; a first selector for selecting the datapacket that has been waiting the longest as indicated by the backlogtracker; and a second selector selecting further data packets forsimultaneous transmission in descending order of waiting time, saidsecond selector skipping data packets requiring a transmit directionincompatible with the direction of transmission of a data packetpreviously selected for transmission at a same time.
 2. The scheduler ofclaim 1, wherein a transmit direction is deemed incompatible by thesecond selector if it can only be created using a directionaltransmission beam that will be fully occupied in transmitting an alreadyselected data packet.
 3. The scheduler of claim 1, wherein a transmitdirection is deemed incompatible by the second selector if it lies too,close in direction to the direction of transmission of a data packetalready selected for transmission at the same time.
 4. The scheduler ofclaim 3, wherein a direction too close in direction to another directionis one in which an angular separation of said directions is less than aminimum value sufficient to avoid mutual interference.
 5. The schedulerof claim 4, wherein the minimum angular separation sufficient to avoidmutual interference is a first separation when the polarizations of thetransmission used in each direction are the same, and a secondseparation when the polarizations of transmission used in each directionare different.
 6. A scheduling method for selecting data packets from abuffer memory for transmission by a phased array antenna comprising atleast one directive beam, in a time-division reuse pattern, comprising:determining a subset of directions in which said directive beam can betransmitted in a next transmission; mapping a destination identifiercode stored with each said data packet to a corresponding beam directionand selecting only data packets whose destinations map to a directionwithin said subset of directions; ordering said selected data packetsaccordering to how long they have been stored in said buffer memory,with said selected data packets which have been stored the longestordered first; and transmitting the selected data packet that has beenstored the longest first by directing the beam to the directionassociated with that data packet, in response to the ordering.
 7. Thescheduling method of claim 6 further including the step of deleting theselected data packets from the buffer memory which have beentransmitted.
 8. The scheduling method of claim 6 further including thestep of setting an indication flag in the buffer memory in associationwith the selected data packets already transmitted.
 9. The schedulingmethod of claim 6, further comprising the step of releasing memorylocations in the buffer memory where the selected data packets that havebeen transmitted were stored so as to provide storage capacity for newdata packets.